Making steam for gasifying coal



United States Patent 3,441,394 MAKING STEAM FOR GASIFYING COAL John C. St. Clair, Madison County, Ohio (Box 333, RR. 2, London, Ohio 43140) No Drawing. Filed May 2, 1967, Ser. No. 635,376 Int. Cl. Cj 3/46; C101) 57/12 U.S. Cl. 48-202 2 Claims ABSTRACT OF THE DISCLOSURE A method for using impure water for making steam for gasifying coal, by heating molten lead by a furnace, contacting and heating superheated steam, containing 0.1% added hydrogen, by the molten lead, reheating the molten lead from the preceding step so that it can heat additional superheated steam, contacting the heated superheated steam with impure water so that the impure water is evaporated at a temperature at which any salts from the impure water that are present in the superheated steam are present in a dry form, removing steam that has been evaporated from the impure water from the larger body of superheated steam and using the removed steam to gasify coal, and reheating the larger body of steam from the preceding step by contacting it with heated molten lead so that the superheated steam can evaporate more impure water.

In converting coal to gaseous products containing methane and hydrogen large quantities of steam are needed for the reaction of hot carbon with steam. Since the steam is not recovered as pure condensate like in a power plant this requires that water be continuously supplied to the plant for making the steam needed. It is highly desirable that the steam be produced at high pressures. First, since many gasification processes are preferably carried out at high pressures, high pressure steam may be required. In the case of coal gasification processes that use steam at lower pressure it is desirable to first produce the steam at high pressure and run the steam through a turbine producing power before the steam is used by the coal gasification process.

However the big trouble with making high pressure steam is that very pure water is needed to prevent highly undesirable scale from forming on the inside walls of the boiler tubes. The making of high purity water from ordinary water is not cheap and the cost for the amount required is a considerable expense in the case of a coal gasification plant.

Another appreciable expense of a coal gasification plant is that usually about half of the steam used in the gasification of coal appears in the product gas unused. In the past it has been the custom to just condense this unused steam and Waste the heat it contains. The condensate contains many objectionable compounds like phenols which in extremely small amounts can cause objectionable flavors in drinking water taken from the stream of water downstream. The disposal of this condensate is going to become a serious problem if many plants are built for gasifying coal.

The ideal solution to the above problems is to devise a system whereby impure water can be used as the feed water to a high pressure steam boiler. The water can be roughly purified before it is used but it obviously cant be cheaply purified to the standards now used for high pressure steam boilers. For instance the product gas can be washed with a small amount of sodium carbonate solution, before it is condensed, to remove about all of the HCN gas and the phenols. This solution can be regenerated at low temperatures by passing CO gas through it and extracting out the phenols with solvents. Feed water ice that has directly contacted the steam containing product gases and has been heated by the steam can be extracted with benzene and the hydrocarbons dissolved in the heated feed water can be mostly removed. However it is not possible to remove all the impurities in the heated feed water without considerable expense. It is the purpose of this invention to provide a very cheap method for using impure water for making high pressure steam, in a steam boiler, suitable for gasifying coal.

In my invention molten lead is heated in tubes in a furnace. Molten lead has a very high rate of heat transfer from it to the tubes through which it is circulated. In the data given in Liquid-Metals Handbook published by the Atomic Energy Commission, June 1, 1959, page 14, it is shown that lead when passed at a velocity of 11.6 ft./sec. through l-inch tubes has a heat transfer coeflicient of 4050 B.t.u./(hr.)( F.)(ft. This is high enough to permit very high rates of heat transfer from very hot furnace gases to the molten lead. If we assume a F. temperature rise of the lead as it circulates through the tubes before the heat in the lead is removed for making steam, it takes 2.1 horsepower to circulate enough lead through 100 feet of l-inch tubing to obtain 500,000 B.t.u. per hour. From a practical standpoint this means that it will take less than 4% of the steam produced to provide the power to pump the lead through the pipes heating the lead and the contactor used to contact the lead with the superheated steam.

The corrosion of the steel tubing used to heat the molten lead, by the molten lead at the temperatures required is negligible. The maximum temperature of the lead will be of the order of 850 F. or about 450 C. According to Corrosion of Steels in Liquid Bismuth and Lead by J. A. James et al., Journal Iron & Steel Insti tute (London) 194, 319-23 (1960), the corrosion of flowing molten lead on steel at 700 C. is roughly 0.01 inch per year. Since the corrosion of steel by lead decreases with lower temperatures the operation at 450 C. should give negligible corrosion.

After the lead is heated it is directly contacted with and heats superheated steam containing enough hydrogen to keep the steam from reacting with the lead. The quantity of hydrogen necessary to keep steam from reacting with lead is small. At 400 C. or 752 F. there is only needed about 0.0001% by volume hydrogen in the steam. (From data in Vacuum Metallurgy by Bunshah, Reinhold Publishing Corp., N.Y., 1958, page 62.) However to make sure there will always be enough hydrogen present I usually prefer to add about 0.1% by volume hydrogen to the steam.

After heating the superheated steam the molten lead is returned to the furnace for heating again. After the superheated steam is heated by the lead impure water is sprayed into the superheated steam, the water evaporating into more steam. The superheated steam is cooled by the evaporation of the water and some of the superheat is removed from the steam. The steam produced by this evaporation is used as product steam for gasifying coal. The rest of the steam is reheated by molten lead for use again in making steam.

It is necessary to keep the temperature of the superheated steam high enough so that the salts dissolved in the feed water which get into the superheated steam are present as powders. This normally just requires that the temperature of the superheated steam be kept only a few degrees above the steams boiling point at that pressure. The only exception to this is when sodium or potassium chlorides or potassium carbonate are present. With sodium chloride the vapor pressure of the steam should be kept below atmospheres and the temperatures above 375 C. For potassium chloride the pressure of the steam should be kept below 110 atmospheres and the temperatures above 400 C. For potassium carbonate the steam pressure should be kept below 70 atmospheres and the temperature above 425 C. If potassium carbonate and any alkali metal sulfides are present in the feed water it is desirable to add a trace of sulfuric acid to the feed water before the usual removal of dissolved gases from the feed water. Then after removal of the dissolved gases the feed water is neutralized by a little lime. In this way the undesirable potassium carbonate and the alkali sulfides are changed to sulfates which are very easy to convert to dry powders. (The preceding data were obtained from Pressure-Temperature Curves in Some Systems Containing Water and a Salt by George W. Morey and W. T. Chen, Journal of the American Chemical Society, 78, 4249-52 (1956). The article contains the needed data on a large number of salts.)

It should be said that my invention in some respects resembles the old Loefiler steam boiler. (See Steam Power Stations by 'Gaffert, McGraw-Hill Book Co., N.Y., second edition, 1940, page 222.) In the Loefiler boiler superheated steam was circulated through the boiler tubes and to the heated steam the feed water was added removing some of the superheat of the steam and evaporating the water before the steam was passed through the tubes of the furnace again. This generated steam with the impurities in the feed water being precipitated out as a powder in the superheated steam. As predicted the LoefHer boiler could use feed water with impurities in it with none of the impurities forming scale on the boiler t-ubes. However the rate of heat transfer from the boiler tubes in the furnace to the steam circulating through the tubes was not high enough for rapid heating of the steam. Also it would be impossible for the feed water to have any organic impurities in it since they would crack and form sooty deposits on the tubes inside surfaces. However by the use of molten lead circulated in the tubes I greatly increase the rate at which the furnace can be operated. Also by keeping the steam and its small amount of vaporized organic material away from the hot tubes I prevent the organic material from forming soot on the inside of the tubes.

The steam produced by my invention will contain of course a little hydrogen. Since the steam is to be used for the gasification of coal in which hydrogen in large quantities is produced the presence of hydrogen in the steam has no disadvantages. The steam will usually contain a trace of silica and it is desirable to remove the silica, which will exist dissolved in the steam, by passing the steam through a nickel packed column in which strong NaOI-I water solution is circulated if the steam it to be expanded through a turbine with the generation of power before the steam is used for gasifying coal,

In conclusion I may say that I have disclosed a method in which I have eliminated the large expense of providing high purity water to make high pressure steam that it to be used in gasifying coal. Also I have recovered the heat in a large amount of steam, that is normally wasted, to heat the water used for producing steam. I have also provided a very cheap method for using the condensate, from gasifying coal, for the water to make steam out of. In this way I have eliminated what threatens to be an expensive pollution problem.

I claim:

1. A method for using impure water for making steam for gasifying coal which comprises: heating molten lead by a furnace, contacting and heating superheated steam by said molten lead, reheating said molten lead from the preceding step so that it can heat additional steam, contacting said superheated steam with impure water so that the impure water is evaporated at a temperature at which any salts from the impure water that are present in the superheated steam are present in a dry form, removing steam that has been evaporated from the impure water from the larger body of superheated steam and using said removed steam to gasify coal, and reheating the larger body of steam from the preceding step by contacting it with heated molten lead so that the superheated steam can evaporate more impure water, said steam contacting the molten lead containing sufficient hydrogen to prevent the steam from reacting with the molten lead.

2. A claim according to claim 1 in which the hydrogen in all the steam mentioned is over 0.01% by volume of the steam.

References Cited UNITED STATES PATENTS 414,601 11/1889 Stevens. 1,872,883 8/1932 Byrne 2O138 XR 3,032,482 5/1962 Shoemaker 203-'100 XR 3,242,975 3/1966 Kogan 203.100 XR 3,376,204 4/1968 Tidball 15920 XR 3,382,917 5/1968 Rice 122-32 XR 1,592,861 7/1926 Leonarz 48204 OTHER REFERENCES Galfert, Steam Power Stations, 4th ed., 1952, pp. 256 258.

NORMAN Y'UDK-O'FF, Primary Examiner.

D. EDWARDS, Assistant Examiner.

US. (:1. X.|R. 

